Polymer microspheres refer to high polymer materials or polymer composites in the geometric shape of a sphere or near-sphere with a diameter ranging from nanoscale to micronscale. Polymer microspheres, due to their particular sizes and structures, have been playing a special and important role for a long time in areas such as electronic information, bio-medical technology, coating materials, etc.
In the coating area, besides its use for improving the coating property (mechanical property, electromagnetic property and so on), microspheres may also be used for adjusting the surface properties of coatings, such as optical properties including light extinction and light diffusion, abrasion resistance, scratch resistance, weather resistance, etc. In the application of photocurable coatings with high solid contents, due to rapid curing of the coatings, filling microspheres can hardly diffuse timely onto the surface of the coatings, often impairing their effect significantly.
Polymer microspheres may be synthesized by means of polymerization, such as emulsion polymerization, precipitation polymerization, dispersion polymerization, etc. These polymerization processes all need to be carried out in a reaction medium (for example, organic solvents like alcohols and ethers). Carbon dioxide fluid is a “green” solvent, having advantages of being non-toxic, environmental friendly, non-flammable, easily available at a low price, recyclable, etc. The use of carbon dioxide fluid in place of traditional organic solvents as a reaction medium for synthesizing polymers can greatly reduce emission of volatile organic compounds (VOC). Being used as the reaction medium for preparing polymer microspheres is an important application of carbon dioxide fluid, the research on which has made many significant progresses. However, the traditional polymerization reaction needs to be conducted under the condition of heating (e.g., 50° C. or higher), and the reaction temperature is higher than the critical temperature of carbon dioxide (31.3° C.). Therefore, carbon dioxide must be in a supercritical state, and accordingly, a higher reaction pressure (>200 bars) is generally required. The resulting high equipment cost and slow reaction process restrain actual application of carbon dioxide in preparing microspheres.